Electronic ballasts, due to its small form factor, light weight, less power consumption, and stable light beams, have become the mainstream of fluorescent lamp ballast. Basically the electronic ballast is a combination of circuits that converts alternating current (AC) into direct current (DC) and then from DC back to AC. More specifically, one of the conventional electronic ballasts converts the AC voltage from the mains into a DC voltage, and then converts the DC voltage, through high frequency oscillation, into a high frequency, high level AC voltage to excite the fluorescent lamp. As shown in FIG. 1, the conventional electronic ballast contains a bridge rectifier circuit 10, a DC filter circuit 12, a high frequency oscillation circuit 14, and a lamp circuit 16. For the sake of simplicity and cost reduction, the DC filter circuit 12 usually only contains a filtering capacitor C1.
The bridge rectifier circuit 10 that rectifies an input AC voltage to charge and discharge the filtering capacitor C1 and a DC voltage with a ripple is thereby developed across the filtering capacitor C1. Because the AC voltage Vs can charge the filtering capacitor C1 only around the crest and trough of its waveform where it has a large enough voltage, the input AC current Is therefore has an impulse waveform. Moreover, in order to reduce the ripple of the DC voltage (i.e. to enhance the filtering effect), usually a capacitor with a large capacitance is used as the filtering capacitor C1. This, however. causes the impulse waveform of the input AC current Is to become even acuter.
FIG. 2 is a waveform diagram showing the input AC voltage Vs and current Is of the conventional electronic ballast. As shown in FIG. 2, the input AC current Is has a seriously distorted impulse waveform. The acute impulses cause an increase in the amount of harmonics (especially the third order harmonics) and a reduction of power factor. The increase of harmonics intensifies electromagnetic interference. If a large number of such electronic ballasts are used simultaneously, there is a high possibility to cause a tripping of the power supply system or even a fire accident in the worst case. On the other hand, a reduction of power factor would increase the power consumption of the power supply system and therefore the power bill as well.
A reduction in the capacitance of the filtering capacitor C1 could indeed abate the distortion of the input AC current Is, reduce the amount of harmonics, and improve the power factor. The DC voltage developed across the filtering capacitor C1, however, would have a more fluctuant ripple. This in turn causes the crest factor of the current of the lamp tube 17 (the peak value divided by the effective value of the lamp current) to exceed the normal rating and thereby reduce the lifespan of the lamp tube 17. In summary, for the conventional electronic ballasts, reducing input AC current harmonics/increasing power factor and reducing lamp current crest factor are contradictory to each other.
Most, if not all, of the commercially available electronic ballasts, even though usually branded as “high efficiency,” commonly have a total harmonic distortion ≧10%, power factor ≈0.5, and lamp current crest factor ≧1.7. In other words, these so-called “high efficient” electronic ballasts actually have a high amount of harmonics and a rather low power factor. The term “high efficiency,” therefore, actually refers to the high frequency lamp lighting. To achieve the true high efficiency, a correction circuit must be added in the electronic ballasts to overcome the foregoing limitations and disadvantages of the conventional electronic ballasts.
Currently, to reduce the amount of harmonics of the input AC current and to increase the power factor at the same time, there are generally two types of correction circuits: the active ones and the passive ones. The active power factor correction circuits adopt active elements and therefore have a complex structure, bulky form factor, and a higher cost. The passive power factor correction circuits can only achieve limited improvement and therefore have little value in real-life applications.